FR2591736A1 - System for measuring the linear displacement of a cylindrical rod such as a thruster cylinder rod, thruster cylinder adapted for such a measurement and method of manufacture thereof - Google Patents

Abstract

This measurement system consists in providing the rod 2 with a graduation of pitch n formed by a spiral or a succession of parallel rings giving a succession of lines of thickness n/2, namely "black" lines 4 absorbing light and bright lines 5 reflecting light and in associating with this rod 2 a displacement sensor 11 comprising at least two direct-reflection optical detectors 13, each comprising a light-emitter element and a photodetector element, the width of the light beam emitted by each emitter element being less than the width n/2 of a line, and the light beams emitted being phase-shifted with respect to each other by an angle equal to pi /p - p being the number of optical detectors which the displacement sensor 11 comprises. According to a preferred embodiment, each optical detector is formed by a pair of attached optical fibres, one optical fibre being used as the emitter element and the other optical fibre being suitable for receiving the light beam reflected by the bright rings 5 of the graduation.

Description

SYSTEM FOR MEASURING THE LINEAR DISPLACEMENT OF A ROD
CYLINDRICAL SUCH AS A CYLINDER ROD, ADAPTED CYLINDER
FOR SUCH A MEASUREMENT AND ITS MANUFACTURING METHOD
The present invention relates to a system intended to allow the measurement of the linear displacement of a cylindrical rod and in particular of a jack rod, as well as a jack adapted to such a measurement.

 Currently, the problem of measuring the displacement of a jack rod is an important problem in particular due to the development of hydraulics and of the so-called "proportional" tire.

 Different systems have been proposed for carrying out such a measurement.

 For example, in FR-2 216 467, it is proposed to introduce into the body of a jack a screw with large pitch, fixed in translation, and turning in a nut fixed on the jack rod, so that the moving it causes the screw to rotate. The number of turns made by this screw is recorded by a sensor, which allows to know the displacement of the cylinder rod.

 It has also been proposed in FR-2,513,370 to place permanent magnets on the cylinder rod so as to detect the displacement thereof.

 Finally, other principles of displacement measurement are based on capacitive, potentiometric measurements.

 All these measurement systems are very expensive to implement, on the one hand, because they generally require a body and an actuator rod specifically adapted to the envisaged measurement system and, on the other hand, because they involve the use of sophisticated measuring devices, such as, for example, in FR-2 204 792, which requires a permeameter.

 These systems also function generally only in the absence of rotation of the jack rod and means must be provided (Cf. FR-2 203 948) to avoid rotation thereof.

 Finally, in the case of a magnetic measurement system, the addition of permanent magnets on the rod can modify the hardness characteristics of the latter as well as its surface condition, which is completely detrimental in particular as regards the service life of the seals associated with the piston rod.

The object of the present invention is to remedy these incon
come and provide a system for measuring the linear displacement of a
rod and in particular a cylinder rod, which is of low cost and which
does not in particular impose any internal modification of the jack body, which
can be used with standard steel cylinder rods, which allows
precise measurement, even when the piston rod is rotated, and which
preserves all mechanical characteristics and in particular the characteristics
surface finish and hardness of the piston rod of the cylinder.

This object is achieved in that the measuring system according to
the invention consists in providing the rod with a graduation of pitch n formed by
a helicoid or a succession of parallel rings giving a succession
n / 2 width lines, namely "black" lines absorbing light and
"brilliant" lines reflecting light, and to associate with this rod a
displacement sensor comprising at least two optical detectors with
direct reflection, each comprising a light emitting element and a
photodetector element, the width of the light beam emitted by each
emitting element being less than the width (n / 2) of a line, and the
emitted light beams being offset from one another by an angle
equal to ff, p being the number of optical detectors that the
displacement sensor (n).

Each optical detector can be formed by a pair of fibers
optics placed side by side, one emitting the light beam and
the other receiving the light beam reflected by the bright rings of the
graduation; at this time, a common light source can be provided
for all transmitting optical fibers. Optical sensors can
also be formed by the association of a photodiode and a photodé
guardian.

Such a displacement sensor is known, for example, from
EP.84 420 204.4 in the name of the Applicant and allows, thanks to the phase shift,
its light sources, to have a precision multiplied by four (in the
case of two photodiodes) compared to the accuracy of the graduation. So,
for example, for a graduation step equal to 0.4 mm, the precision obtained
will be 0.1 mm with two optical detectors whose output signal is
transformed into square wave. For more explanations, see the
description of the above mentioned EP.84 420 204.4.

The association of such a displacement sensor with the cylinder rod,
therefore makes it possible to measure the displacements carried out by the latter without requiring any modification of the internal structure even of the cylinder body and using a relatively simple modification of the cylinder rod.

 In addition, in the case where the graduation is formed by a succession of parallel rings, an identical reading is obtained whatever the angular position of the jack rod, which therefore eliminates the risks of error linked to a rotation of the latter.

 According to a preferred embodiment, the graduation on the piston rod is achieved by etching it at the location of the "black" rings and filling these engravings with a black annular deposit having a hardness at least equal to that of material constituting the cylinder rod.

 The etching will preferably be carried out by photochemical process and the deposition of black material by electrolytic means.

In any case, the invention will be well understood and other characteristics will be highlighted with the aid of the description which follows with reference to the appended schematic drawing representing by way of nonlimiting examples several embodiments of the measurement system of displacement according to the invention and the method of manufacturing the cylinder rod
Figure 1 is an exploded perspective view of a jack equipped with the displacement measurement system according to the invention, according to a first embodiment
Figure 2 is a sectional view of the displacement sensor of Figure 1
Figure 3 is a sectional view of a second embodiment of the displacement sensor mounted around the cylinder rod; ;
Figure 4 is an end view of Figure 3
Figure 5 is a view similar to Figure 3 of a third embodiment of the displacement sensor
Figure 6 is an end view of Figure 5
Figure 7 is a perspective view of a mounting device which can be used for achieving the graduation of the rod
Figures 8 to 13 are, on an enlarged scale, schematic views in half-section illustrating the different stages of a first method of producing the graduation of the jack rod
Figures 14 to 17 are views similar to Figures 9 to 13 illustrating the different stages of a second method of achieving the graduation of the cylinder rod.

 FIGS. 20 to 20 are views similar to FIGS. 9 to 13 illustrating the different stages of a third method of producing the graduation of the cylinder rod.

 Figure 1 shows an actuator 1 equipped with the displacement measuring device according to the invention. As shown in this figure, the rod 2 of this jack is provided with a graduation 3 which is formed of alternately black 4.5 and “shiny” 4 4.5 parallel rings 5.

 These rings 4,5 have a width equal to n / 2 (n, not of the graduation) identical and extend perpendicular to the longitudinal direction of the cylinder rod. The width of each of these rings 4.5 is equal, in this case, to 0.2 mm for a graduation step of 0.4 mm, but it is obvious that the step can have any value.

 Around the cylinder rod 2 is mounted the device 11 for measuring the displacement of the cylinder rod 6 itself. This device is mounted on a ring 7 which has, in this case, a parallelepiped shape and which is provided at each of its angles with a hole 7a allowing it to be threaded on the tie rods 1a of the jack and to be fixed at the end of this jack by means of nuts 8. Of course, any other method of fixing the ring on the jack can be provided.

 As shown more particularly in FIG. 2, this ring 7 also has a central bore 9 of diameter adjusted with gentle friction to that of the rod 2 of the jack, so as to allow movement thereof, this bore with friction adjustment Soft also makes it possible to automatically obtain the desired focal length for the optical detectors during industrial mounting on the cylinder rod.

 This ring 7 also has at its upper end a cylindrical housing 10 allowing the positioning of the displacement sensor 11 which is externally in the form of a cylindrical sleeve. The bottom of this housing 10 is connected to the bore 9 by a bore 12. Inside the displacement sensor 11 are housed two optical detectors with direct reflection 13 of the type known under the trade name "Leds 1000". these optical detectors 13 comprise a light-emitting diode, emitting a thin practically parallel light beam with a width equal to approximately 0.2 mm, and a photodetector directly receiving the light beam emitted by the associated diode and reflected by the shiny parts 5 of the graduation 3, and capable of emitting an electrical pulse on each passage on a graduation 5, that is to say each time a light beam is reflected by the shiny parts 5, this electrical signal being transformed, in known manner per se, in a square wave and being amplified by appropriate means integrated in the sensor II. Means for the cumulative counting of the electrical pulses emitted by each photodetector are also mounted in a manner known per se outside the displacement sensor 11.

 Two scraper seals 14 are provided in the bore 9 of the ring 7, at the end ends thereof, so as to wipe the cylinder rod 2 during its movements. In this way, the optical detectors 13 are in an area between two seals 14, and are therefore in an area constantly wiped and protected from dirt and projections, which could disturb the reading (oil, dust, shavings, ...) .

 One of the seals 14 intended to be placed on the side of the jack could possibly be deleted; in this case, the optical detectors 13 would still be in a sealed zone between the seal of the jack ensuring the seal between the rod and the body of the jack and the scraper seal 14.

 The distance between the bottom 10a of the housing 10 and the adjacent upper end of the bore 9 is approximately 4.5 mm, which corresponds to the focal distance of the photodetectors 13. When the displacement measuring device is put in placed on the cylinder body, the cylinder rod 2 can move inside the bore 9 and the graduations 4,5 which it carries are "seen" by the optical detectors 13 via the bore 12 .

The operation of this displacement sensor is as follows
The light beams emitted by the diodes of the optical detectors 13 are phase shifted by an angle equal to Tr / 2. In this way, the same graduation 5 will be "seen" by the two photodetectors 13 at an interval of 90, so that the signal resulting from the two signals given by the photodetectors of each detector 13 will comprise, over the same period, four pulses regularly distributed, each pulse corresponding to a rising edge or a falling edge, which therefore makes it possible to multiply the precision of the graduation by four. Thus, in this case, the precision obtained will be 0.1 mm, since the graduation pitch is 0.4 mm. In addition, the phase shift of the light beams makes it possible to discriminate, in a manner known per se, from the using doors or the like, the direction of movement of the rod 2 relative to the jack 1. For further explanation, reference may be made to the description of EP.84 420 204.4 previously mentioned.

 It will be noted that the ring 7 has the advantage of being able to adapt the displacement sensor to several diameters of actuator rods since it suffices to modify the diameter of the bore 9 and the depth of the housing 10 while keeping the distance, between the bottom 10a of the latter and the bore, equal to the focal distance of the optical detectors, ie 4.5 mm as indicated above.

 Of course, each of the optical detectors described above could also be formed by a pair of optical fibers, one "emitting" conducting the light beam coming from a source and the other "receiving" receiving the light beam reflected by graduation or any other known optical detector. In this case, the operation would be identical and a common light source could be provided for all of the "emitting" optical fibers.

 It will be noted that the displacement sensor thus obtained is particularly solid and resistant.

 In addition, with substantially the same mounting, that is to say an apparatus formed by a cylindrical body and a sliding rod inside, and provided with the graduation 3 described above, but without the supply of fluid, a measuring device (of the "bicycle pump" type) can be obtained which can be fixed at each of its ends, on the one hand, to a mobile object whose displacement can be measured and, on the other hand , at a fixed point of reference. Thus, each movement of the movable object will result in a corresponding displacement, which can be measured, of the rod of the device. Such a measuring device can advantageously be used for measuring the displacement of mobile elements such as levers, connecting rods, etc., on which it is not possible to fix a displacement sensor of conventional type.

 Figures 3 to 6 show other examples of mounting the displacement measuring device on the cylinder, where the shim and the detector are integrated in one piece for the needs of mass production.

 In the case of FIGS. 3 and 4, each of the two optical detectors 13 is mounted in a cylindrical sleeve 15 provided with a central bore 15a of diameter adjusted to the diameter of the jack rod 2. As shown more particularly in FIG. 4 , the two optical detectors 13 are mounted angularly offset by approximately 300, but so that their median axes are located substantially in the same perpendicular plane of the jack rod 2. In fact, their median axes are offset by an equal value at a quarter graduation, or 0.1 mm, in the longitudinal direction so as to achieve the phase shift of the light beams. The sleeve 15 is fixed in any manner known per se to the cylinder body 1 and can for example be simply fixed to the tie rods 1a of this cylinder using nuts, as indicated above for the ring 7.

 In the case of FIGS. 5 and 6, each of the two optical detectors 13 is mounted in a cylindrical sleeve 25 provided with a central bore 25a of diameter adjusted to the diameter of the jack rod 2.

Unlike the previous case, the two detectors 13 are not placed so that their axes are substantially in the same transverse plane of the cylinder rod, but that they are, on the contrary, aligned along a generatrix thereof. this.

 In both cases, one of the two detectors 13 is mounted in the sleeve 15,25 by means of a ring of elastic material 26 on which a screw 27 extending longitudinally relative to the direction of the sleeve 15 is supported. , 25. This screw 27 makes it possible to slightly modify the position of the optical detector 13 and therefore to adjust the phase shift between the two detectors 13, the latter having to be separated by a distance equal to a multiple of the pitch of the graduation increased by n / 4. This same principle of adjustment can be used for the device of FIGS. 1 and 2.

 FIGS. 7 to 20 illustrate three possibilities of making the graduation 3 on the jack rod 2, this having to be carried out without modifying the surface state of the rod, or altering the hardness thereof.

 FIG. 7 firstly shows a mounting device which can be used for making the graduation in the two cases illustrated in FIGS. S to 17. This mounting device is formed by a lathe 30 on which the piston rod 2 to be fitted with the graduation 3 is mounted between points. A laser 31 is mounted on the transverse carriage 32 of the lathe. This laser 31 is positioned so that its light beam 33 strikes the jack rod 2, in a direction perpendicular thereto. The actuator rod 2 is driven by the rotation of the turning head with a rotational movement indicated by the arrow 34, while the carriage 32 moves transversely step by step in the direction indicated by the arrows 35. step of this carriage 32 is equal to the step of the graduation is 0.4 mm, while the laser beam is adjusted so as to have a thickness of 0.2 mm, which therefore corresponds to a half step or the width of a graduation 4.5.

 It is easily known that by rotation of the jack rod 2, the sun will be exposed by means of the laser using rings of width equal to 0.2 mm and that the desired graduation 3 is ie the series of rings 4.5 will be produced by successive displacements of 0.4 mm from the carriage and sunstroke only every 0.4 mm.

 Of course, with an appropriate advance of the carriage 32 carrying the laser 31, that is to say a continuous movement thereof, the cylinder rod 2 will be exposed according to a helical rod, thus defining graduation lines according to not wanted.

 According to a first method illustrated by FIGS. 8 to 13, the graduation 3 of the jack rod 2 is carried out in a photochemical manner.

First of all, the cylinder rod 2 which is coated with a layer of hard chrome 40, which has a hardness equal to approximately 750 Vickers and which has a thickness of approximately 12 microns, is coated with a layer of varnish photosensitive 41 over its entire external surface. This coating with varnish is carried out in a manner known per se by soaking the cylinder rod 2 in a bath of this varnish or by spraying.

 The rod 2 thus coated with the protective varnish is placed on the device shown in FIG. 7- and is exposed by rotation and displacement of the laser 31 as explained above on parallel rings 43 of width equal to 0.2 mm and spaced from 0.4 mm. The parts 43 of the photosensitive varnish having been exposed, they are then removed by suitable pickling carried out for example by soaking in a pickling bath and then form annular grooves 43a. The configuration shown in FIG. 9 is obtained in which the piston rod 2 is coated with the hard chrome layer 40 and also comprises rings 44 of photosensitive varnish, each offset by 0.4 mm and separated by annular grooves 43a.

 This piston rod 2 is then chemically attacked by dipping in an acid suitable for the metal constituting the coating of this rod.

 We then obtain the configuration shown in Figure 10 in which the coating 40 of the piston rod 2 is hollowed out according to the rings 45 corresponding to the void 43 left by the varnish which has been exposed and removed and in which the parts located between the engravings 45 in the form of a ring are still covered by the layer of photosensitive varnish 44 and are therefore intact. The depth of the etching 45 produced is of the order of five to fifteen microns.

 The next step consists in removing the protective layer of photosensitive varnish 41 still in place on the rings 44. This pickling is carried out in the same way as described above by soaking in a pickling bath. At this moment, the piston rod has the configuration shown in FIG. 11, that is to say that it has only one layer 40 of general thickness - equal to twelve microns and etched according to rings 45.

 During the next step, the parts engraved in the form of rings 45 are filled with a black material 46 of hardness at least equal to that of the layer 40 coating the piston rod. This black material can for example be black chromium and it is then deposited electrolytically. I1 can also consist of titanium oxide (black) and is then deposited by evaporation.

 This filling of the engravings 45 can also be carried out using a black resin charged with black alumina powder, this filling then being carried out by extrusion, scraping of the overhanging parts and polymerization. Finally, it is possible to envisage filling the engravings 45 with hard, black alumina, this filling being carried out by selective deposition of aluminum in the engravings, then oxidation so as to obtain alumina and coloring.

 Currently, filling with hard black chromium mentioned first is the preferred solution because, for a very reasonable cost, it makes it possible to obtain a hardness greater than 650 Vickers. The thickness of the layer of black deposited is at least equal to the depth of the etching, namely five to fifteen microns. The configuration shown in FIG. 12 is therefore obtained, in which the piston rod is coated uniformly with a layer of uniform thickness, namely both on the non-etched parts 44 and on the etched parts 45.

 It will be noted that this layer has waves 47 whose recesses 47a correspond to the engravings 45 and that the thickness of this layer must be sufficient for the bottom of the recesses 47a to arrive at least at the level of the external surface of the coating layer 40 .

 The last step in carrying out the etching is a rectification of the piston rod 2-to the initial diameter, namely the outside diameter of the coating layer 40, so as to obtain the desired finish H7. The configuration of FIG. 13 is then obtained, in which the piston rod 2 has a succession of rings 44 formed by the coating metal itself of the piston rod and black rings 46 formed by black hard chrome set up in engravings 45.

 Due to its rectification, the piston rod has a perfectly smooth surface condition and therefore not likely to damage the seal of the cylinder. In addition, it does not have a lower hardness zone since its hardness is equal to at least 650 Vickers everywhere.

 Figures 14 to 17 illustrate a second method of graduating the cylinder rod.

 In this case, the cylinder rod 2 is treated before application of its hard coating of chromium 40 and one therefore starts from a rod of common steel and which can be engraved more easily than a material such as hard chromium having a hardness of 650 Vickers. .

 In this case, the engravings 55 of the jack rod 2 can therefore be produced directly on the device 30 using the laser 31. They can, of course, also be produced in a photochemical manner as explained for the first method. These engravings will however be of a double depth compared to that of the engravings 55 produced in the methods described above and will have a depth of approximately twenty microns.

 The second step of the process shown in FIG. -15 consists in coating the piston rod 2 thus graduated with a first hard layer 56, for example of hard chromium, this deposit being produced in a manner known per se by electrolytic means, and on a thickness of about ten microns.

 In the third step of this process shown in FIG. 16, a layer of black hard chrome 57 or the like having a thickness equal to ten microns is deposited on the first layer 56 deposited, this layer 57 also being deposited electrolytically.

 Finally, the last step shown in FIG. 17 consists in rectifying the piston rod to a diameter corresponding to its original diameter increased by - twice the thickness of the first deposited layer 56, so as to obtain the state of desired surface as well as the configuration shown in FIG. 17, in which the piston rod 2 has rings 58 formed by the first layer of glossy coating and black rings 59 formed by the second layer 57 of black coating.

 As before, the piston rod obtained has an excellent surface condition and constant hardness over its entire periphery.

It will be noted that this method is particularly advantageous compared to the first method described, since it requires only four stages, while the first method comprises seven stages.

 Figures 18 to 20 illustrate yet another method of scaling the cylinder rod.

 In this case, the steel used for. the cylinder rod 2 is a heat-treated steel, the mechanical properties of which, in particular the hardness, can be modified by an appropriate heat treatment (quenching, etc.).

 As shown in FIG. 18, the first step of this method consists in producing etchings 65, similar to those 45 in the case of the first method, these etchings 65 being able to be produced in a photochemical manner as described above or using 'a laser 31 of sufficient power.

 During the second step of this process, - shown in FIG. 19, the engravings 65 are filled with a layer 66 of black material, capable of withstanding the heat treatment which will be carried out on the jack rod, this deposition is also carried out electrolytically.

 The next step is the actual heat treatment, which will be carried out in such a way as to give the desired hardness to the cylinder rod 2.

 These last two steps can also be reversed, that is to say that the heat treatment of the actuator rod 2 is carried out before deposition in the engravings 65 thereof of black material (but after etching of course); at this time, the material filling the engravings can be formed by hard black chromium, titanium, etc. as in the first process described.

 Finally the last step of the process, shown in Figure 20, is the rectification of the piston rod 2 to its original diameter, so as to obtain the desired surface state.

 It goes without saying that the present invention is not limited to the embodiments described above by way of non-imitative examples; on the contrary, it embraces all of the variants using similar or equivalent means.

Claims (14)

- CLAIMS -  1. A system for measuring the linear displacement of a cylindrical rod such as an actuator rod, characterized in that it consists in providing the rod (2) with a pitch scale (n) formed by a helicoid or a succession of parallel rings giving a succession of lines of width n / 2, namely "black" lines (4) absorbing light and brilliant lines (5) reflecting light and to be associated with this rod 12) a sensor for displacement (11) comprising at least two direct reflection optical detectors (13), each comprising a light emitting element and a photodetector element. the width of the light beam emitted by each emitting element being less than the width (n / 2) of a line, and the light beams emitted being phase shifted relative to each other by an angle equal to Tpr, p being the number of optical detectors that the displacement sensor (II) comprises.  2. Measuring system according to claim 1, characterized in that each optical detector is formed by a pair of contiguous optical fibers, one optical fiber serving as a transmitting element and the other optical fiber being able to receive the light beam reflected by the shiny rings (5) of the graduation.  3. Measuring system according to claim 1, characterized in that each optical detector is formed by a photodiode and a photodetector capable of receiving a light beam reflected by the bright rings (5) of the graduation (3).  4. Measuring device according to one of claims 1 to 3, characterized in that it is formed of a cylindrical body -and a rod (2) provided with the scale (3) and mounted slidingly at inside of the cylindrical body and in that it is capable of being fixed by each of its ends, on the one hand, to a fixed reference point and, on the other hand, to a moSile element ,. whose displacement we want to measure.  5. Device for measuring the linear displacement of a cylinder rod according to the measurement system of claim 4, characterized in that the optical detectors are mounted on a sleeve (7,15,25) fixed to the end of the body cylinder and surrounding the cylinder rod (2) and in that means (26,27) are provided for adjusting the phase shift of the optical detectors.  6. A method of producing the graduation of a rod for the purpose of measuring its displacement using the measuring system according to any one of claims I to 4, characterized in that it consists in producing on the periphery of the rod and at regular intervals corresponding to the half-step of the graduation, helical or ring-shaped engravings (45,55) and to fill these engravings (45,55) with a black material of hardness at less equal to that of the rod (2) to its area (40).  7. Method according to claim 6, characterized in that the filling of the engravings (45,55) with a black material is carried out by electrolytic deposition of thickness at least equal to the depth of the engravings (45,55) and in that the excess material is removed by rectifying the rod to its original diameter.  8. Method according to one of claims 6 or 7, characterized in that it comprises the steps consisting in  - coating the rod with a layer of photosensitive protective varnish (41)  - insulating the protective varnish (41) according to rings (43) perpendicular to the longitudinal direction of the rod (2);  - remove the exposed parts (43) of the protective varnish (41) by pickling  - chemically etching (45) the rod (2) at the location of the uncoated parts of the rod (2);  - remove the non-exposed parts (44) of the protective varnish by stripping  - then fill the engravings (45) with black hard material.  9. Method according to claim 6, characterized in that it comprises the steps consisting in:  - Directly etching the rod (2) before depositing on it a hard coating (56), according to a propeller or rings (55), the depth of which is substantially equal to the sum of the thickness of the layer of hard coating (56) and layer of black hard filler (59) of the engravings  - deposit a first layer of glossy hard material (56)  - deposit a second layer of black hard material (57)  - Grind to the diameter corresponding to the initial diameter of the rod increased by twice the thickness of the layer of hard material (56).  10. Method according to one of claims 6 or 7, characterized in that it comprises the steps consisting in  - engrave the rod (2) directly before heat treatment thereof using a propeller or rings (65)  - deposit a layer (66) of black hard material  - heat treat the rod (2), so as to obtain the desired surface hardness.  11. Method according to one of claims 6 or 7, characterized in that it comprises the steps consisting in  - engrave the rod (2) directly before heat treatment thereof using a propeller or rings (65)  - heat treating the rod (2) so as to obtain the desired surface hardness  - deposit a layer (66) of hard black material.  12. Method according to any one of claims 6 to 11, characterized in that the black hard material is formed by black hard chromium and in that it is deposited by electrolysis.  13. Method according to any one of claims 6 to 11, characterized in that the filling of the engravings (45) is carried out by selective deposition of aluminum, oxidation to alumina and coloring.  14. Method according to any one of claims 6 to 11, characterized in that the filling of the engravings is carried out by extrusion of a resin loaded with black alumina powder, scraping and polymerization.